WASHINGTON — President Obama stepped into the space Wednesday where the Rev. Dr. Martin Luther King Jr. once stood, summoning his dream of a colorblind society in a celebration of 50 years of progress and a call to arms for the next generation.

On a day of misty rain, tens of thousands of Americans — black, white and every shade in between — returned to the site of King’s “I Have a Dream” speech to listen to the nation’s first black president pay tribute to the pioneers who paved the way for his own ascension to the heights of U.S. government.

“Because they kept marching, America changed,” the president said as King’s family watched. “Because they marched, a civil-rights law was passed. Because they marched, a voting-rights law was signed. Because they marched, doors of opportunity and education swung open so their daughters and sons could fin
ally imagine a life for themselves beyond washing somebody else’s laundry or shining somebody else’s shoes.

“Because they marched,” he added, “city councils changed and state legislatures changed and Congress changed and, yes, eventually, the White House changed.”

The symbolic journey from King to Obama on the steps of the Lincoln Memorial animated the 50th anniversary of the March on Washington for Jobs and Freedom more than any oratory. While Obama’s line about the White House changing was his only reference to his unique place in history, the power of his presence was lost on no one.

But it also illustrated the challenge to a movement to reframe its mission for a new era. With an African-American in the Oval Office, it is harder to argue about political empowerment than it was in 1963, and much of the day’s message centered on tackling persistent economic disparity and newer frontiers of civil rights, such as equality for gay men and lesbians.

Several speakers, including former President Carter, tied the historic nature of the event to controversies of the moment, including the Trayvon Martin case, New York’s police-frisking policy and the Supreme Court ruling this summer that overturned part of the Voting Rights Act.

“I think we know how Dr. King would have reacted,” Carter said.

Yet former President Clinton said that, for all of the current challenges, Americans have never had more opportunity to shape the future if they can put aside their differences.

“It is time to stop complaining and put our shoulders against the stubborn gates holding the American people back,” he said.

Also on hand were Caroline Kennedy and Lynda Johnson Robb, daughters of the two presidents most associated with civil rights, as well as a phalanx of celebrities such as Oprah Winfrey, Bill Russell, and actors Jamie Foxx and Forest Whitaker.

Some lions of the civil-rights era also were there, including Rep. John Lewis, former Ambassador Andrew Young and the Rev. Joseph Lowery — grayer, thicker, slower, but stirring emotions of their youth.

Many members of King’s family also participated, including his 85-year-old sister, Christine King Farris. The Rev. Bernice King, his daughter, noted that women were largely missing from the speakers list in 1963, but they were a significant presence 50 years later. She delivered a rousing call to “let freedom ring” before the ringing of a bell saved from the 16th Street Baptist Church in Birmingham, Ala., where a bombing by the Ku Klux Klan killed four girls just weeks after King’s speech.

At 52, Obama is too young to remember the march, and, raised in Hawaii as the son of a white mother and a father from Kenya, his own connection to the civil-rights movement is more distant than, say, that of his wife, Michelle, whose parents grew up in segregated America. But he clearly felt the pressure to live up to the moment, noting that “no one can match King’s brilliance.”

Obama used his 29-minute address to argue for a renewed mission to ensure that opportunity is available not just for a few but for the many.

He was to a large extent preaching to the choir. This was a crowd of supporters, many from the Washington area, and to them it was a day to reflect not only on King’s legacy but also on Obama’s. Those old enough to remember the march marveled at a black president’s standing where King had.

“If you say that’s not the fruition of the dream, I don’t know what is,” said Bill Carr, a licensed clinical social worker from Montclair, N.J., who is black.

Nearby stood Bill Tate, a retired engineer who is white and who wore a button from the original march, when he was a student at the University of Maryland.

“Who would have guessed 50 years ago that in less than 50 years we would re-elect — re-elect! — a black president?” he asked. “No one can deny that we haven’t made some progress.”

If the 1963 march had an air of freedom and spontaneity, the event Wednesday felt at times choreographed and forced. When Young addressed the crowd, he did so in song, delivering a moving rendition of “Woke Up This Morning With My Mind Stayed on Freedom.” But when he implored the crowd to join in, the few who did could barely be heard.

“We’re not here to declare victory,” Young later told the crowd. “We’re here to simply say that the struggle continues.”